1. Field of the Invention
This invention relates to the field of loading liquefied gases into cylinders.
2. Prior Art
Typically a filling station has a large storage tank in which a cryogenic substance is stored in liquid form. Portable cylinders, which are superinsulated to maintain the cryogenic substance in its liquid form, must be periodically refilled from these filling stations and transported to a place of use.
During the transfer of liquefied gases from the storage tank to the portable cylinder, a portion of the product gas is wasted. These filling losses, depending on the circumstances, may be a significant percentage of the product gas.
A number of prior systems have attempted to deal with these large filling losses. These systems include recirculating systems to prevent loss of flashed vapor, top filling the cylinder with pumps and pump aided transfer systems. None of these have been entirely satisfactory.
The recirculating systems have recirculated the flashed vapor generated when the liquid from the tank has entered the cylinder. Recirculating the flashed vapor back to the tank can result in a no loss system. However, there has been a serious risk of contamination of the tank if a contaminated liquid cylinder has been filled. Also the heat absorbed by the recirculated vapor is added to the storage tank, an undesirable event. Further, a sophisticated operator has been required to run this system.
Top filling with a pump generally has operated only under ideal conditions in which the plumbing between tank and cylinder is precooled and the liquid cylinder is cold. Under typical conditions the cylinder must be blown down periodically to avoid losing pump prime or damaging the seals. Further, the operation takes 10 to 12 minutes on average and requires a sophisticated operator to deal with pump problems and maintenance.
It has been known to transfer cryogenic substances from a storage tank to a liquid cylinder using pressurized transfer filling and centrifugal pump filling. In pressurized transfer filling the pressure head within the storage tank has been used to force substance through pipes into a cylinder. In centrifugal pump filling, a centrifugal pump has been disposed in line between the storage tank and the liquid cylinder for transferring substance.
The cylinder which has been filled includes two connections associated with filling, an inlet port and an outlet vent. Substance has been loaded into the cylinder through the inlet port while the outlet vent was left open allowing any liquefied gas which returns to a gaseous form to vent to the atmosphere. As substance flowed through a filling station the substance absorbed heat causing the substance to change state into gas and causing high venting losses due to excessive flashing from the pressure letdown between storage tank and cylinder pressure as a substance entered the cylinder.
U.S. Pat. No. 4,475,348 discloses the use of back pressure in a cylinder to decrease filling losses. The outlet vent of the cylinder being loaded was adapted to provide a predetermined amount of back pressure within the cylinder. The pressure of the tank and the pressure of the cylinder were monitored and the pressure of the cylinder was adjusted to maintain a single differential pressure of 10 psi for all filling station configurations and for all product gases. This method decreased filling loss to some degree but its effectiveness varied as the configurations of the filing stations varied and as the type of product gasses varied.
It has also been known that during centrifugal pump transfer of substance from a storage tank to a cylinder, centrifugal pumps have been subject to cavitation. Cavitation was caused when the cryogenic substance absorbed thermal energy causing the substance to vaporize in the pump inlet and bubbles of the vapor to be carried to the impeller of the pump. The pump rotor then spun more rapidly in the gas bubble since the gas offered much less resistance than the liquid. This rapid spinning caused friction and heat which warmed the gas further causing further vaporization. Unless the motor was stopped when this occurred, the pump motor could burn out or the casing or rotor of the motor could break due to internal friction. If the substance being loaded is liquid oxygen, there was a high potential for a safety hazard.
Rattan in "Cryogenic Liquid Service", Chemical Engineering, Apr. 1, 1985, page 95 discloses bleeding a small liquid stream through a hole in a pump to keep the pump cool to deal with this problem. However in very hot areas a large amount of substance must be wasted by this method. Another method disclosed in this same article, is bringing the pressure within a system up to a level that prevents flashing.
Another danger present when liquid cylinders were loaded with a cryogenic substance was that when the cylinder was overfilled, liquefied gas product was discharged from the outlet vent of the cylinder. It was common in the prior art to continue filling a cylinder until liquefied product was discharged from the outlet vent as a way of determining when the cylinder was full. In addition to wasting product this can be dangerous since the liquefied gas may injure an operator by cryogenic burns or asphyxiation or cause an explosion or a fire.